139QMB 52.4mm with 24mm carb problems

[guszec]

Hello all of You, I'm new here. I have a generic 12 inch 139qmb scoot (Motom Gipsy, Moto- Magnus Capro style). I've mounted this config on it:

Naraku 52.4mm kit
Malossi Multivar variator
Naraku variator counterplate
Naraku torque driver
Naraku clutch bell
Naraku Twist Control
Naraku crank
Top Racing adjustable friction
Naraku jet flow spacer
Generic chinese sport camshaft
Handmade exhaust pipe with 25mm diameter (exhaust original)
Pollini Airbox air filter
Red clutch springs
White central spring
6 or 7gr rollers, I don't remember correctly


The ignition coil and cdi remained original. The intake manifold is a chinese generic for 125/150 engines.

The problem is- i have two 24mm carbs for it. One is generic chinese, and one is Naraku. The scoot runs better on the chinese one, but I cannot find right adjustment spots. For Naraku it goes like this:

-The needle needs to be all way up. If I lower it the scoot loses power immediately.
-Main jet above 98 will cause it to loose ignition on high speeds (85kph)
-Main jet smaller than 94 will cause it to loose some top speed
-The mixture screw is REALLY sensitive. It came already like that. I can screw it all the way down and the scoot will not die. But going over 1/8th of a turn from full closed will already cause the engine to run poorly! So I have full carb regulation within a REALLY small range. The best result i can get is that it tops like at 95/100kph with fatique.
-Has no problem idling

For the generic chinese it goes like this:

-Needle like naraku (but it has 3 steps instead of 5. Behavior is the same)
-Main jet above 108 will cause it to loose ignition on high speeds (85kph)
-Main jet smaller than 92 will cause it to loose some power and speed
-The mixture screw is not that sensitive, but is rough to turn sometimes and does not seem to do much. Screwed all the way down does significantly reduce RPM, but it does not die. The best result I can get is up to 110kph sometimes even. The engine seems to make less fatique and vibrations.
-Has problem idling on cold


I live in like 400m altitude. I don't know what to do anymore. The one thing I must do is to alignment of the intake manifold as it is like 1mm off the centre. But this does not explain the strange behavior of carbs. I don't know what can I do anymore with them. Maybe I'll try to cut the central spring in the carb? Anyone has any idea that could help me?

I'd like to know if someone had similiar setup, and would give me an idea on jetting the thing? The main jet as well as the other one. And I would like to know the probable cause of engine still idling with the mixture screw all way in?

Also- an recommended RPM meter. I have none currently. Bought some crap from china, but threw it out. At stable idle it gave 700- 1900RPM with the engine running smoothly


Thank You, have a nice road ahead

[GrumpyUnk]

I will venture a guess. A 24mm carburetor seems to be larger than usable on a 138QMB. A larger carb will flow more CFM/CMM than can be used. Given the flow would be in the lower operating range, the carb is finding it difficult to modulate the fuel:air flow and becomes finicky.
Most seem to use 18-19mm on their scooters. The 19mm variety seem to be questionable, the vendors will bore the outlet diameter to 19mm, but the throttle plate/butterfly is actually still an 18mm. You must check.
Given that carburetors work on pressure/vacuum/airflow and speed through the venturi, when you shift the volume demanded to the lower end of the functional range, it will not work as well.
Personal opinion is that a 24mm is too large for even an 80cc. I had a Sym Classic 150 that I think had a 24mm, as was the carb on the CSC SG250 I had. Sold due to getting too bleeping old and having moments of imbalance.
Others may disagree, so good luck.
tom

[guszec]

Hi there, thanks for the reply! The thing is- the scoot has 52.4mm kit, so it's 90cc. Nonetheless it does make sense- the 24mm carbs are used for 125 and 150 chinese engines. But this is what the big bore kit manufacturer recommends, so... I feel my head spinning from this lol.

[GrumpyUnk]

As noted, it is up to you. My main point was with large bore and low end volume flow, you are at the 'low' end of the carb's ability to mix properly and function without being finicky. IMO.
A slide carb should work as well as a vacuum-slide carb, again, IMO.
If your idle circuit is clogged, you will not be able to get a good idle or mid-range. You will end up with the mid-range trying to handle idle speed and it just cannot meter very well as it is below its function point(I think), so it will be VERY touchy. If the pilot is clogged, it may just fall on its face after starting out, may scrape through to higher speed, etc.
If the capacity is not somewhere close, the carb will not perform as well as a better-matched carb.
tom

[guszec]

In that case I'll try to obtain an native 19mm carb and I'll see how it behaves. Thank You for the tip, I think I'll update this thread when I buy one. I see that there isn't much informations on such builds.

[sinfull]

1st thing I always check is the vacuum nipple on the intake is clamped zip tied tightly, vacuum leaks cause a lot of problems. Slide carbs and most carbs don't need vacuum to run, so check all the hoses and make sure they don't actually do anything besides vent float bowl or just look cool. Chinese carbs usually have a lot of hoses that do nothing. Only vacuum needed is usually for vacuum fuel pump if you have one.

[GrumpyUnk]

Why or how does a vacuum operated fuel pump work when the throttle is wide open? In theory, with WOT, there should be no manifold vacuum, thus making the pummp inoperative. So, how does it work?
I am sure someone knows.
tom